Last night, an unknown buyer paid $142 million for Irish-born British painter Francis Bacon's "Three Studies of Lucian Freud."

With a "hammer price" of $127 million plus a $15 million commission, it's a nominal record for a work bought at auction.

Bacon is a strong candidate for greatest painter of the 20th century, having escaped the orbit of the pure abstract expressionists to create his own visceral mode of painting portraits.

Still, given the debate over rising inequality in the U.S. and elsewhere, paying $142 million for a work of art may seem ... extreme.

And that's probably true.

However, the mechanics of the art world suggest the amount is justifiable.

First, as a triptych, the buyer is actually getting three paintings for the price of one. Plus, for a decade after their initial 1970 exhibition the works were displayed separately, apparently to Bacon's great frustration, according to The Wall Street Journal's Kelly Crow. Today the piece is one of two complete triptychs Bacon made of Freud, another world famous artist (and grandson of Sigmund Freud) who was heavily influenced by Bacon. (Both are now dead by the way, and, grimly, death helps drive up value). And it apparently took a lot of effort to get all three canvases in one place. So a premium has been placed on reuniting the works.

Meanwhile, the previous record for a contemporary work sold at auction was also a Bacon: In 2008 (an ominous year, to be sure), Roman Abramovich paid $86.3 million — $94 million in real dollars — for a triptych (simply titled "Triptych") of a more recent vintage (1976). Works often decline in value the later they came in an artist's career.

Scale can also affect price, and each of the canvases in "Freud" are 6 ft. tall. And the brightness of a painting can also drive up its value, and the top half of "Freud" glows with orange and yellow.

Finally, the Christie's team juiced the sale of the work by moving it up in the auction lot at the last minute, sensing strong demand. The opening price was set at $80 million and the first bid came in at $100 million. Art Market Monitor's Howard Rehs has discussed the pricing momentum the right work can generate through 1% groupthink:

"What really makes an artist’s market viable is the amount of support it has, or: how many players are in the game? In addition, how many of those players will always be there to make sure the price levels stay up? The more support, the more stable a market becomes. ... When you have tens of millions, or even hundreds of millions, invested in an artist’s work you do not want to see values go down.

The world set a record this year for total number of billionaires. Among the very rich, at least, this appears to be a sign the earnings accumulated in the last few years are not going anywhere.